This invention relates to a method of constructing and installing offshore structures.
Until recently, offshore structures have been steel, have been constructed on shore, have been launched in water, and have been towed to their site of operation in a horizontal floating position by a shallow draft vessel.
However, recently, in the North Sea gravity concrete offshore structures have been introduced, and these have ushered in a completely different construction technique. These gravity concrete structures are much heavier than steel structures and have required sheltered construction sites where the water is more than two hundred feet deep. This depth is needed both during construction and also during the full length of the route along which the completed structure is towed to its site of operation. The contemporary method of constructing such concrete offshore structures includes building a lower watertight portion of the structure in a dry dock. The dry dock is located adjacent to the sea and has a construction floor that is below sea level. The floor level is determined by the draft necessary to float and move the partially completed structure to a deep water wet dock, where the upper portion is added and the structure completed. The requirement that there be a wet dock imposes a difficult condition for the production of gravity concrete structures, even where such wet docks are available or can be made. Such construction facilities are found in very few places, mostly in the fjord region of Norway. Therefore, in other parts of the world, including the United States, where such facilities are lacking, the construction must be carried out without a deep water dry dock or a wet dock, and this has limited the use of concrete structures.
An object of the present invention is to make it possible to construct major concrete offshore structures without using a deep water dock. The structure, or a large portion of it, may be constructed directly on a barge embodying this invention and then later transported to the operating site and launched, without the offshore structure having to leave this construction vessel until the moment it is actually launched. The barge serves as a shallow draft construction dock, as a barge for transporting the object to be constructed to the operating site, and also as a mechanism for launching the offshore structure by submerging the barge and then moving it out from beneath the offshore structure.
Another object of the invention is to make it possible for offshore structures of concrete to be constructed in water depths of no more than 40 or 50 feet, while the prior-art wet-dock type of construction would require a water depth of 200 feet or more for the same structure. Similarly, the present invention enables transport or towing of such structures through water that is only 40 or 50 feet deep, whereas the prior art required a depth of 250 or more for the same structure.
Another object of the invention is to enable more rapid transportation of the offshore structure. When the offshore structure is towed in a deep-draft floating position, the towing is very slow, and exposure and risk are heightened because of the greater length of time during which storms may arise. In the present invention it becomes possible to shorten the time for transporting and launching the offshore structure at its site and to lessen the risks and exposures on the open sea.